Fast Direct Oxidation of All Alkanes in Soil by Hydrophilic Fe-SOM

ABSTRACT Conventional Fenton chemical oxidation was more difficult to directly oxid all alkanes of long, mid and short-chain alkanes in oil-contaminated soil. To investigate the direct and fast oxidation mechanism of all alkanes in the soil by the fast oxidation group using two different soils for the direct oxidation experiments. The results showed that the hydrophilic Fe-SOM rich in hydrophilic groups could directly oxidize all alkanes in the soil with a high removal rate of total petroleum hydrocarbons (TPH) (59.0%–72.5%). Similarly, for the oxidation of long-chain alkanes (C23-C30), the fast oxidation group in which the hydrophilic Fe-SOM was located had higher removal rates (52%-64%). Regarding long chain oxidation per unit hydroxyl radicals (•OH) intensity, the fast oxidation group was significantly higher than the slow oxidation group (1.3–4.2 times). The results revealed a well-linear fit (r2 = 0.97, 0.81) for total iron and FeOOH with hydrophilic groups such as C=O and C-O-C in the fast oxidation group regarding content. Hydrophilic Fe-SOM enabled the rapid decomposition (51% − 88%) of H2O2 in the presence of hydrophilic promoters such as humic acid (49%−79%). The hydrophilic groups and hydrophilic promoters made the •OH generated by decomposition collide and react with pollutants rapidly to maximize the •OH in the liquid phase. The TPH removal efficiency per the unit intensity of •OH in the fast oxidation group was 3.74 times higher than in the slow oxidation group. The long-chain removal efficiency was 4.2 times higher than that in the slow oxidation group. The above results indirectly proved that the oxidation reaction in the fast oxidation group occurred at the petroleum pollutants and also demonstrated that the technology is an in situ oxidation technology. This technology can directly oxidize all petroleum hydrocarbons in the soil and has excellent engineering application value. GRAPHICAL ABSTRACT


Introduction
Growing energy demand and innovations in oil extraction technologies have rapidly increased crude oil extraction, refining, and use globally (Yuan et al. 2018).Because petroleum's mobility and permeability made the soil less fertile, with altered physicochemical properties and high human hazards, petroleum hydrocarbon-contaminated soil must be treated (Kiamarsi et al. 2020;Usman, Hanna, and Faure 2018;Zhen et al. 2021).Fenton oxidation, a kind of in situ chemical oxidation (ISCO), has become an economical and efficient remediation technology, which referred to the production of •OH by using the solution of H 2 O 2 and Fe 2+ (Laurent et al. 2012;Usman et al. 2022).Due to the high oxidation potential of •OH(Eo = 2.80 V) (Cheng et al. 2016;Ma et al. 2018), nonselected oxidation of total petroleum hydrocarbons (TPHs) in soil could be achieved (Paixão et al. 2020;Usman and Ho 2020).Moreover, previous reports indicated that needle ferrite (FeOOH) as high-potential iron could also lead to Fenton reactions with H 2 O 2 , showing excellent oxidation efficiency (Pervez et al. 2021;Tsai et al. 2009).Compared to bioremediation, chemical oxidation can oxidize TPHs in soil within a few hours to achieve the desired remediation effect (Huguenot et al. 2015;Martínez-Pascual et al. 2015;Pan et al. 2020).
Because the octanol-water distribution coefficients (log Kow) of short-chain alkanes were less than medium and long-chain alkanes.(DM Mackay et al. 2006).It suggested that short-chain alkanes were relatively more hydrophilic and easily oxidized first by •OH, which can be confirmed in our previous study and others.Sivagami et al. ((Sivagami et al. 2019)) found that by Fenton oxidation of long-chain alkanes (C 21 -C 30 ), removal was only (13%).Pardo et al. (Pardo et al. 2014) also found that the removal of C 18 was only 2% in the citric acid trona chelate system; our previous study had also reported only 7% degradation of long-chain alkanes (C 25 -C 40 ) at higher H 2 O 2 concentrations (2.9 M) (Xu et al., 2011).Previous studies had shown that oil-absorbing iron bound to the organic matter could transfer •OH to the solid phase and complete the oxidation of long-chain alkanes under the conditions.However, this view was from the component of each chain hydrocarbon and at a relatively low TPH content (4000 mg/kg) (Xu et al. 2019).The above studies indicated that the current technology could not directly oxidize all alkanes in the soil.
Previously it was investigated that FeOOH can combine with C-H and C=O to form a target for the targeted oxidation of alkanes in soil (Xu et al. 2020).Accordingly, it was known that FeOOH and functional groups in the soil might form a specific structure that affected the catalytic of H 2 O 2 .Therefore, the exact mechanism of the direct oxidation of alkanes remains uncertain.Moreover, the hydrophilic nature of humic substances had been demonstrated by studies such as that of Yang et al. (Yang and Antonietti 2020), who showed that humic acids could bind hydrophilic molecules and strong ion binding.Scott et al. concluded that quinones in soil humic acids might affect the redox processes of organic matter in the soil (Scott et al. 1998).From the above studies, it was hypothesized that humic acid-like substances in the soil could be involved in oxidation as hydrophilic promoters that facilitate the accelerated run-up of liquid phase •OH to the surface.This study aimed to create an iron material with a predominantly high-potential FeOOH occupancy in two different soils by optimizing the proportion of iron material based on previous research.The linear correlation of this iron material with the C=O and C-O-C functional groups in the soil suggested that these two functional groups played essential roles in the fast oxidation reaction.Also, we knew that both functional groups had hydrophilic properties, according to the study by Nima et al. (Nima et al. 2020).With the assistance of hydrophilic promoters such as humic acid, this structure accelerates the oxidation process of hydrogen peroxide.This Fe-SOM structure was hydrophilic, and the oxidation reaction was the fast oxidation group.The experiments were carried out to regulate the different dosing ratios of humic acid and chitosan (2%, 10%) and to control the percentage of functional groups in the organic matter of the two soils and the iron structural morphology to prepare eight iron materials.The effect of direct oxidation of all alkanes by the fast oxidation group was demonstrated by analyzing soil iron content, soil metal composition, soil iron morphology, soil organic matter distribution, total organic carbon content, functional group composition, and TPH removal rate.In addition, the rapid decomposition characteristics of H 2 O 2 were investigated, and the relationship between the direct oxidation of all alkanes removed by the fast oxidation group was analyzed.Ultimately, the fast oxidation group elucidated the mechanism of the direct oxidation of all alkanes in the soil.
Crude oil-contaminated soil was prepared by mixing the above soil containing iron materials with crude oil for oxidation experiments.Spiked crude oil was extracted from an oil well in Yan'an, diluted with dichloromethane and stored at room temperature (22 ± 2°C).The short-chain alkanes (C 11 -C 14 ) accounted for 15.33% of the n-alkanes, the short and medium-chain alkanes (C 15 -C 18 ) accounted for 23.29%, the medium-chain alkanes (C 19 -C 22 ) accounted for 20.84%, the medium and long chain alkanes (C 23 -C 26 ) accounted for 20.08%, and the long chain alkanes (C 27 -C 30 ) accounted for 20.47%.The specific components were shown in Figure S1, and the specific reagents used in this experiment can be found in Text S1.

Preparation of hydrophilic Fe-SOM
Previous studies had found that increasing the decomposition time of H 2 O 2 in the Fenton reaction can influence the oxidation efficiency and affect the removal of TPH (Xu et al. 2019).In this study, we used S1 and S2 with different contents of soil organic matter and organic bound state iron as substrates and added organic matter for modification.S1 soil properties were high organic matter (C=O, C-O-C absorbance of 0.02, 0.364), the iron component was FeOOH, the Fe-SOM obtained under this modification method is hydrophilic Fe-SOM and the oxidation reaction was the fast oxidation reaction group (the highest primary reaction rate was 0.22), referred to as the fast oxidation group.The nature of S2 soil was low organic matter (C=O, C-O-C absorbance of 0.011, 0.171), the iron component was FeOOH, the Fe-SOM obtained under this modification method is non-hydrophilic Fe-SOM, whose oxidation reaction was the slow oxidation reaction group (the highest primary reaction rate is 0.07), referred to as the slow oxidation group.
Previous studies showed that Cs and HA had a colloidal effect, allowing Fe 2+ to chelate with SOM in the soil.However, no attention was paid to its effect on the humic acid content of the soil and whether it was hydrophilic, which in turn affected the decomposition of H 2 O 2 , •OH production and the removal of TPH.Within this study, we conducted optimization experiments by weighing 20 g of soil material S1 and S2 into 250 ml conical flasks and then adding different ratios of Cs and HA at 2:0, 10:0, 0:2 and 0:10 ratios.Finally, distilled water and H 2 O 2 (30% concentration) were added to maintain a total liquid volume of 120 mL (i.e., final Fe 2+ concentration of 5.8 mol/L and final H 2 O 2 concentration of 0.90 mol/L).At the same time, adjusted the pH to 7.5 with 5 mol/L NaOH and mixed well.When the concentration of Fe 2+ in the solution no longer changed, the combination of Fe 2+ and organic functional groups was completed (about 7 days).After the reaction, the supernatant was poured off and washed several times by centrifugation with ultrapure water (18.2MΩ/ cm, ELGA, UK), and then the cleaned soil samples were dried in a freeze dryer (FD-1D-50; Boycon, Beijing).The iron content and fractions (total iron) formed in the solid phase of the soil samples were determined.The organic functional groups and the relative content of organic matter in the iron materials were analyzed using Fourier infrared, three-dimensional fluorescence (3DEEM) and X-ray photoelectron spectroscopy.The metal structure in the soil was scanned using the X-ray diffractometer.

Hydrophilic Fe-SOM oil dyeing experiment
Soil samples with 5 g of hydrophilic Fe-SOM or non-hydrophilic Fe-SOM were mixed with soil samples in a fume hood by adding crude oil at appropriate concentrations diluted with dichloromethane.Ultimately, dichloromethane was evaporated to obtain crude oil-contaminated soil samples of 11,940 mg/kg for S1 and 11,047 mg/kg for S2.Each experiment was done three times.
After oil dyeing, the oxidation experiment was carried out with the iron-containing soil sample.The specific steps of the oxidation experiment were: weighing 5 g of iron-containing crude oil contaminated soil sample into a 150 mL triangular conical flask, then adding 54.6 mL of distilled water and 5.4 mL of H 2 O 2 (maintaining a 60 mL system), the H 2 O 2 concentration was 0.90 mol/L.The pH was also adjusted to 7 with 5 mol/L NaOH, and the oxidation experiments were carried out at room temperature (22 ± 2°C).Hydrophilic Fe-SOM and non-hydrophilic Fe-SOM were used to catalyze the H 2 O 2 .A set of blank control with distilled water only was made simultaneously.The decomposition of H 2 O 2 was measured by the iodometric method (about 48 h), and the concentration of TPH and each chain hydrocarbon remaining after GC measured the reaction.To study the oxidation mechanism, the above oxidation experiments were repeated.In every Fenton reaction, the residual H 2 O 2 concentration was measured regularly, and the rate of decomposition of H 2 O 2 was calculated.Ultimately, the instantaneous intensity of •OH was measured by the electron paramagnetic resonance (epr) method (bruker a300).Three parallel samples were made for each experiment.

Hydrophilic Fe-SOM leaching experiment
After the oxidation experiment, 0.5 ml of the supernatant of the mixed solution after the reaction of the fast and slow oxidation groups were taken separately and added to a 50 mL colorimetric tube.Added 1 mL each of 1 + 3 hydrochloric acid and 10% hydroxylamine hydrochloride to the colorimetric tube, diluted to 15 mL with distilled water, then added Congo red test paper and shaken to make the Congo red test paper sink to the bottom of the solution.At this point, added a small amount of saturated sodium acetate solution so that the test paper just turned red, added 5 mL of buffer solution, and finally added 2 mL of color developer (0.5% o-phenanthroline solution), fixed the volume to the 50 mL marke with distilled water, mixed well, and after 15 min of color development, used UVmini-1240 to determine the absorbance at a wavelength of 510 nm, and then brought in the drawn standard curve to calculate the concentration of iron in the solution at this time.The experimental results showed that the Fe concentration in the supernatant of the two groups was low (0.001 mg/L, 0.0013 mg/L), and a small amount of Fe was leached out from the experiment.

Analysis of iron content in fast and slow oxidation group
The iron material obtained by the four methods described above was dried and weighed to 2 g using the Tessier method (Tessier, Campbell, and Bisson 1979) to extract exchangeable iron, carbonate-bound iron, iron oxide (manganese) bound to iron, organically bound iron and residual lattice-bound iron in that order.Operations were performed in agreement with previous studies (Xu et al. 2018), and the specific procedures were described in supporting material Text S2.In addition, analytical calculations for the slow oxidation group were consistent with those for the fast oxidation group.In this study, the distribution of Fe species in the eight soil materials were shown in Table 1.

Determination of organic functional groups by Fourier transform infrared spectrometry (FTIR)
To investigate the relationship between functional group absorption peaks and FeOOH as well as total iron, infrared spectra were determined by FTIR (IS50; Nicolet Instruments, USA).The samples were dried in an oven at 60°C, mixed KBr with a ratio of 1:100, and powdered in an agate grinding body before determination.Measurements were made at a scan number of 32 and a transmittance of 4 cm −1 .The functional group absorbance was measured in the main sample bin by pressing the tablets; then, the functional group positions were determined by automatic baseline correction and the peaks.At last, the integral was calculated in the 500-4000 cm −1 .The Nicolet IS50 PC/IR software package processed all infrared spectral data.SPSS statistical package on IBM/PC was applied for principal component and cluster analysis (Fan et al. 2014).

Analysis of oxygen-containing functional groups by Boehm titration
The Boehm titration method determined the oxygen-containing functional groups on hydrophilic and non-hydrophilic iron surfaces.Different intensities of bases (C 2 H 5 ONa, NaOH, Na 2 CO 3 , and NaHCO 3 )were used to neutralize the acidic oxygen-containing functional groups of different intensities on the hydrophilic and nonhydrophilic iron surfaces.Experimental details are given in Supplementary Material Text S5.The experimentally obtained C=O, O-H concentrations were presented in Table 3

Determination of relative organic matter content by 3DEEM
After the reaction, the supernatant of the soil sample was taken, passed through a 0.45 μm filter, diluted to a specific multiple, calibrated using ultrapure water, and detected the Raman peak.Samples were added to a 1 cm quartz fluorescence cuvette with four-sided light transmission.The fluorescence intensity was calculated in a Hitachi fluorescence spectrophotometer (HACH F-7000) with the following specific parameters: a 150 W xenon arc lamp; the voltage was 700 V; the bandwidth was set to EX = 5 nm, EM = 10 nm; scanning speed: 12000 nm/min; and the instrument response time was automatic.All the results were based on Chen's method to analyze the percentage of organic matter content in each partition organic matter (Chen et al. 2003).The specific partition and analysis calculation methods were shown in the supporting material Text S3.

Determination of structure by X-ray photoelectron spectroscopy (XPS)
XPS tests for hydrophilic and non-hydrophilic iron are precisely the same as before.Details can be found in Test S4.Values are expressed as mean ± standard error (3 repetitions).

Structural analysis of the fast and slow oxidation groups using X-ray tube diffractometer (XRD)
The two soil powders were finely ground to ensure a homogeneous sample size before detection by the X-ray tube diffractometer (Nayan et al. 2002).Testing angles were 5 to 90° in steps of 0.02° at a speed of 12°/min.

Changes in total organic carbon (TOC) in soils of the fast and slow oxidation groups between periods of oxidation
The sample was filtered using a 0.22 μm filter membrane and the TC and IC (German elements) was measured using the German Elemental Total Organic Carbon analyzer with automatic injection.Before switching on the instrument, ensure that the phosphoric acid, halogen, and desiccant tubes were sufficient.After switching on the instrument, the oxygen cylinder valve was slowly opened so that Press was between 940-950 mbar and the oven temperature was set at 850°C.A standard curve was made using the standard solution and measured samples, with each sample set to be injected three times.The final results were averaged and the total organic carbon (TOC) was the difference between TC and IC, and the samples were calculated from the standard curve for their concentrations.

Method for analysis of H 2 O 2
The concentration of H 2 O 2 in the supernatant was measured using a UV-1240 spectrophotometer provided by Shimadzu after the color development of titanium sulfate (Tsai et al. 2009).Then the Kobs (h-1) of H 2 O 2 decomposition was calculated by Gao's method using a pseudo-first-order kinetic model (Gao et al. 2022).

Method for analysis the determination of •OH
•OH was measured by EPR (Bruker A300, Germany) at room temperature.Samples (20 μl; through a 0.45 μm filter) and 10 μl DMPO (1%) were mixed homogeneously in a 2-ml centrifuge tube.To measure EPR, the capillary tube was aspirated with 1 cm of the liquid mixture, and the bottom end was sealed through petroleum jelly before being placed in a quartz sample tube.Parameters for EPR were as follows: central field = 3600 g; scan width = 100 g (spin capture experiment with 5,5-dimethyl-1-pyrroline N-oxide (DMPO)); scan time = 60 s; g -factor = 2.000000.Receiver gain = 30 db; modulation amplitude = 1.0 g; modulation frequency = 1 g; the number of scans = 3; attenuation = 20 db; the instantaneous intensity of •OH was obtained by calculating the peak height.Finally, the total intensity of •OH was obtained by integration with time, and three parallel samples were made for each set of experiments (Fang et al. 2015).Some of the data from the single test were shown in Figure S5.

Extraction and analysis of TPH
The extraction of TPH was consistent with previous studies (Xu et al. 2020).Crude oil extraction from soil was performed using EPA test method 3550 B. Each soil sample added with 20 mL of dichloromethane was shaken for 12 h, followed by ultrasonic treatment for 15 min and mechanical shaking for 30 min (250 rpm).The crude oil solution was extracted three times and dehydrated with anhydrous sodium sulfate (pre-baked in an oven at 105°C for 3 h).The final volume of the extracted crude oil solution was adjusted to 50 mL (Lindsey et al. 2003).
TPH (1-μL extract) was analyzed using an Agilent HP 6890 gas chromatograph (GC) (Agilent 6890, USA) with an HP-5 column and a flame ionization detector (FID).Intermediate parameters can be found in the supplementary material.The gas chromatography was started and ended with the injection of pure dichloromethane and TPH mixed standards.Proper petroleum standard curves were calibrated using an internal standard (methyl heptadecanoate) (standard oil purchased from Sigma).Calculate TPH and alkane concentrations by comparison with the petroleum standard curve.

Statistical analysis
Analysis of variance (ANOVA) was performed for each group of primary data using the SPSS software package 25.0 to assess the statistical differences between the different protocols.The significance level of the results was 0.05 to determine statistical significance.

Results and discussion
To screen for functional groups and iron structures in soil materials modified with humic acid and chitosan that can directly oxidize various alkanes in the soil in the pre-Fenton oxidation stage.We refer to the reaction carried out by soil S1 as the fast oxidation reaction group, and its formation of Fe-SOM was called hydrophilic Fe-SOM.The reaction carried out with soil S2 corresponding to the same modification method was called the slow oxidation reaction group, and the Fe-SOM formed was what we called non-hydrophilic Fe-SOM.The effect of the two oxidation reactions on removing TPH from the soil, their structural properties and mechanistic studies were further investigated and described below.

Direct oxidation of all alkanes in the soil by the fast oxidation group
As illustrated in Figure S2, the fast oxidation group could oxidize and remove all alkanes directly from the soil.The removal rate of TPH increased from 59.0 to 72.5% when the amount of catalytic iron in soil S1 of the fast oxidation group increased from 4309 to 5915 (mg/kg Fe) by four soil modification methods.In contrast, the TPH removal rate increased from 44.0 to 60.0% when the amount of catalytic iron in soil S2 increased from 3812 to 5129 (mg/kg Fe) in the slow oxidation group, and the TPH removal rate increased relatively low and was consistently lower than that of the fast oxidation group.During the remediation of petroleum-contaminated soil, we often focus on the removal effect of long-chain alkanes and the low removal rate associated with the difficulty of their resolution.We found that the oxidation of long-chain alkanes (C 23 -C 30 , initial value of 4844 mg/kg) in the fast oxidation group was above 2500 mg/kg, and the removal rate of long-chain alkanes could be 52.0%-64.0%,compared with only about 32.6%-48.9% in the slow oxidation group.The fast oxidation group had a higher removal rate for TPH (or long-chain alkanes) than the slow oxidation group under the same modification, which showed that the fast oxidation group directly affected all alkanes in the soil.
By comparing the treatment effect of each component of the chain hydrocarbon and the oxidation per unit of •OH, we found the same pattern (Figures 1, 2).The oxidation of the long chain fraction per unit •OH in the fast oxidation group was 1410 -2918 mg/(kg•a.u.), and the TPH oxidation was 3867.96-8085.7 mg/(kg•a.u.).The oxidation of the long chain  fraction in the slow oxidation group was 601-1082 mg/(kg•a.u.), and the TPH oxidation was 1874.06-4085.14mg/(kg•a.u.), the former being 1.3-4.2times higher than the latter.All the above results indicate that the fast oxidation group can directly oxidize all alkanes.By comparison, the slow oxidation group can only achieve some effect on the short and medium-chain alkanes, but the effect on the long-chain alkanes is minimal.

Characterization of iron morphology in the fast oxidation group
The total iron content of the fast oxidation group was determined by the Tessier method (Table 1), which was higher in the fast oxidation group (4309, 5171, 5309, 5915 mg/kg Fe, respectively) and relatively lower in the slow oxidation group (3812, 4728, 4889, 5129 mg/kg Fe, respectively), under the same modification method.As the above data showed, the iron content of the fast oxidation group was greatly enhanced by the modification.
In addition, the XPS peak areas of the fast oxidation group were analyzed by convolution and Origin integration and corresponded to the binding energy positions of specific iron structures (Figure 3, Table 2).The high-potential FeOOH in the fast oxidation group was determined to be mainly ɑ-FeOOH (713.4 eV) and γ-FeOOH (713.0 eV), and the highpotential FeOOH content was based on the sum of the split peak areas of these two substances.From this, it is clear that the iron valence state existed mainly in the form of Fe (III) during the increment of iron amount, and the Fe (III) content reached more than 85% in the fast oxidation group and the slow oxidation group.The sum of Fe 3 O 4 and FeO content in the fast oxidation group was 12.19%, 13.31%, 0.02%, and 13.93%; the sum of Fe 3 O 4 and FeO content in the slow oxidation group was 12.66%, 2.81%, 11.54%, and 14.26%.It can be seen that the main iron morpho-transfer in hydrophilic Fe-SOM was the conversion of Fe (II) to Fe (III), and the central iron component that carries out the catalytic action was FeOOH, which was consistent with the results of Usman et al. (Usman et al. 2021).

Functional group characteristics of the fast oxidation group
The structural relationships of the functional groups of the fast oxidation group showed that (Table 3) the total amount of iron in the fast oxidation group had a clear linear relationship with the C=O (1900-1700 cm −1 ) and C-O-C (1040 cm −1 ) functional groups (Figure 4a1, a2), which can be seen that the fast oxidation was related to the above functional groups.As the absorbance values of the above functional groups increased from 0.063 to 0.089, from 0.016 to 0.023, and from 0.286 to 0.396, the corresponding iron content increased from 4309 mg/kg to 5915 mg/kg.A positive correlation exists between the absorbance of the functional groups contained and the total iron content in the fast oxidation group (Figure 4a,c).In contrast, there was no significant linear relationship between the content of each functional group and the amount of iron in the slow oxidation group (Figure 4b,d).It is evident that the functional groups C=O and C-O-C can increase the total iron content, in agreement with the study by (Nima et al., 2020).Under the same gradient of total Fe (4889 mg/kg − 5171 mg/kg), compared with the slow oxidation group, the absorbance of functional groups C=O and C-O-C in the fast oxidation group were significantly linear with the total Fe (r 2 = 0.71-0.97)and FeOOH (r 2 = 0.72-0.81),with the absorbance of 0.019 ±  0.0010, and 0.309 ± 0.0155, which were 1.68, and 2.68 times higher than those of the slow oxidation group.We also integrated each functional group and obtained the area of each part.With the same total iron gradient (4889 mg/kg − 5171 mg/kg), the absorbance areas of the functional groups C=O and C-O-C in the fast oxidation group were 5.77 and 1.59 times higher than those in the slow oxidation group, respectively.In addition, the absorbance area of the functional groups (Figure 4e,f) and the amount of iron in the fast oxidation group were significantly linear (r 2 = 0.96).Conversely, no such pattern was observed in the slow oxidation group.Thus, we can judge that the difference in functional group content is the dominant factor in the difference in removal effect between the fast and slow oxidation groups.Since the functional groups C=O and C-O-C are hydrophilic, the •OH and petroleum, initially in solid-liquid non-contact, get promoted and contacted.At this time, the •OH in the liquid phase was guided by the hydrophilic polarity to run to the solid so that the oxidation reaction occurred in the petroleum and directly oxidized the petroleum pollutants in the soil.

Characterization of organic matter in the fast oxidation group
By performing 3DEEM analysis of the organic fractions of the fast oxidation group (Figure 5b1-c4), a significantly higher proportion of humic acids was found in the fast oxidation group (49% − 79%) than in the slow oxidation group (26% − 60%) after regional fluorescence integration (Table 4).It was assumed that the presence of humic acid in the soil, which acted as a hydrophilic promoter, made the above oxidation process more rapid.In addition, the SOM content of S2 soil was 3.75%, while the SOM content in S1 soil was 4.11%.The analysis may be due to the reduced content of hydrophilic promoters such as humic acid due to the low organic matter content, which prolongs the oxidation reaction time.This situation will result in less •OH being available for direct oxidation, resulting in much less effective removal.In summary, the oxidation efficiency and rate of the reaction were significantly improved by increasing the content of hydrophilic promoters such as humic acid.

Changes of total organic carbon(TOC) fraction in the fast oxidation group
Analysis of the TOC content of the soil before and after oxidation (Figure 6).The TOC content of the fast oxidation group increased by 2.7-3.5 times from 576 mg/kg, with a maximum post-oxidation concentration of 2001.6 mg/kg, while the TOC content of the slow oxidation group increased by only 1.0-2.29 times from 580.8 mg/kg, with a maximum post-oxidation concentration of 1329.6 mg/kg.Chen et al. revealed a significant increase in TOC content in petroleum-contaminated soil after ozone treatment (Chen et al. 2017), consistent with the present study.After oxidation in the fast oxidation group, the TOC content was significantly increased, not only in line with the oxidation effect but also to provide nutrients for subsequent biological and plant growth.

Characterisation of H 2 O 2 catalyzed by the fast oxidation group
By analyzing the decomposition of H 2 O 2 concerning •OH intensity characteristics in the fast oxidation group and the slow oxidation group, the decomposition in the fast oxidation group was the fastest from 0 -10 h.To clarify, the Kobs for the fast oxidation group (4309, 5171, 5309, 5915 mg/kg Fe) were 0.22, 0.08, 0.19, and 0.13, respectively.While the Kobs for the slow oxidation group (3812, 4728, 4889, 5129 mg/kg Fe) were 0.05, 0.04, 0.05, 0.07.The former were all higher than the latter, up to 5.5 times higher (Figure S3), by fitting the pseudo primary kinetic equations for 0-10 h at different Fe amounts.As shown (Figure 7a), higher H 2 O 2 decomposition rates (51%-88%) are obtained in the fast oxidation group.The fast oxidation group required 7-18 h to decompose 80% of H 2 O 2 , while the slow oxidation group required around 16-36 h to decompose the same amount of H 2 O 2, as can be seen by the corresponding H 2 O 2 decomposition versus time (Figure 7c).It can be seen that the oxidation group effectively increased the reaction rate of the Fenton reaction.Likewise, we can discover that the TPH oxidation per unit intensity of •OH in fast oxidation group (4309, 5171, 5309 and 5915 mg/kg Fe) was 8085.71、3867.96、4612.16and 4085.14(mg/kg•a.u.−1), respectively, which were 3.72, 2.06, 1.88 and 1.36 times higher than those in the slow oxidation group, by analyzing the unit hydroxyl removal.From the above, the amount of long-chain oxidation per unit intensity of •OH was 1.3-4.2times higher in the fast oxidation group than in the slow oxidation group.According to this analysis, the efficiency of •OH oxidation was generally higher in the fast oxidation group than in the slow oxidation group(Figure 7b).Furthermore, the •OH intensity was lower in the fast oxidation group (0.87-2.12a.u.), while it was higher in the slow oxidation group (2.18-2.59a.u.).The above results indicated a more efficient utilization of •OH in the fast oxidation group, using a lower intensity of •OH to achieve a higher oxidation amount.
In conclusion, the decomposition time of H 2 O 2 in the fast oxidation group was significantly faster than that in the slow oxidation group, saving time costs, analyzed from the perspective of H 2 O 2 decomposition.In terms of •OH usage efficiency, the fast oxidation group can achieve up to 4.2 times higher •OH usage per unit than the slow oxidation group, enhancing the efficiency of •OH usage.It was also verified that •OH and petroleum pollutants reacted at the same location, and there was no waste of •OH generated.At a hydrogen peroxide concentration of 0.9 mol/L, the strength of the hydroxyl radicals produced by hydrophilic iron oxidation can be increased by up to 4.2 times compared to conventional oxidation techniques, and the hydrogen peroxide oxidation time can be reduced by 55%, significantly reducing the time cost of oxidation.The efficiency of petroleum hydrocarbon oxidation removal using hydroxyl radicals was also increased by a factor of 4.2.Notably, the hydrophilic iron oxidation effect was superior to conventional oxidation techniques for the application of long-chain alkanes, with long-chain oxidation rates of 52.0-64.0%.This technology combined high efficiency, speed and economy and was highly utilized in engineering applications.

Metal composition analysis (XRD) in the fast oxidation group
Figures S4 pattern characteristics of the original soil in S1 and S2 after modification.The standard XRD card for SiO 2 in the original soil (PDF47-1144) fitted well with the peaks was evident in the graph and, by comparison with the standard XRD card, indicating that the material present in large quantities in the original soil is silica, which was consistent with reality.There was also a better fit to the XRD standard card (PDF46-1312) for FeO in both sets of soil samples by comparing the other peak shapes with the standard card.This suggests that more FeO was produced in the fast and slow oxidation groups of the soil samples and the results are consistent with the pattern of enhanced FeOOH content in the XPS analysis.Due to the complex soil conditions, other metals did not have standard cards corresponding to them and might not exist in crystalline form.

Mechanistic study of the direct oxidation of all alkanes by the fast oxidation group
The mechanism can be summarized as follows: after organic modification, the hydrophilic Fe-SOM in the fast oxidation group contains abundant hydrophilic groups and catalyzes the reaction in the form of FeOOH as a catalyst in iron form.Dual promotion by hydrophilic groups and hydrophilic promoters reduced the collision path between the oxidant and the contaminant, allowing the •OH in the liquid phase to run more quickly into the solid phase  for direct oxidation with the petroleum hydrocarbon contaminants in the solid phase.This situation would allow maximum usage of the short-lived •OH and enhance its availability.
In addition, plots were made of the TPH removal rate versus the sum of the three components mentioned above.A significant linear relationship (r 2 = 0.96) was found for the removal efficiency of TPH with increasing levels of all three (Figure 4e).As described, H 2 O 2 rapidly and efficiently oxidizes all alkanes in the soil directly by the triple action of the catalyst, the hydrophilic groups C=O, C-O-C and the hydrophilic promoter.The fast oxidation group also showed further improvement compared to others and our previous study (Figure 8) ( The efficiency of •OH utilization was also significantly improved compared to the studies of others.Qin et al. used 4.80 mol/L of H 2 O 2 .Finally, the long-chain oxidation rate was only 28-41% (Qin et al. 2021).In contrast to the present study, where the long chain oxidation rate was 52-66%, the results of the present study in terms of •OH utilization efficiency indicated that the oxidation reaction was carried out at the site of the soil contaminant, avoiding the waste of •OH.
Compared to our previous study, targeted iron was only studied for C-H and C=O functional groups, and the humic acid-like content was only 22.8%-51.6%.In the fast oxidation group, studied for both the hydrophilic groups C=O and C-O-C, the humic acid-like content (49%-79%) was also 1.53-2.15times higher than the humic acid-like content in the target iron.In the presence of hydrophilic promoters (humic acidlike), the •OH can be accelerated to reach the oxidation reaction site directly, reducing the waste of large amounts of •OH and significantly increasing effectiveness efficiency.

Conclusion
The hydrophilic Fe-SOM directly oxidized all alkanes in the soil with a high removal rate of TPH (59.0%-72.5%).In particular, for the oxidation of long-chain alkanes (C 23 -C 30 ), the long-chain oxidation rate of •OH was 1.3-4.2times higher in the fast oxidation group than in the slow oxidation group.Moreover, the decomposition of hydrogen peroxide was more rapid in the fast oxidation group (up to 88% in 10 hours).Similarly, the fast oxidation group had lower hydroxyl radicals (0.87-2.12 a.u.) but higher oxidation, which can be seen in its higher OH oxidation efficiency.The characterization analysis for the fast oxidation group postulates that the FeOOH and hydrophilic groups, such as C=O and C-O-C in the fast oxidation group, form a stable hydrophilic Fe-SOM.This structure rapidly and efficiently decomposes H 2 O 2 in the presence of a hydrophilic promoter, allowing •OH in the liquid phase to reach the petroleum surface directly.This result also indicated that the oxidation of the hydrophilic iron occurred at the same location as the petroleum and that the fast oxidation group could efficiently utilize the •OH and avoid the wastage of free radicals.The slow oxidation group had a higher •OH intensity (2.18-2.59a.u.), but the removal was significantly less effective, reflecting the slow oxidation group's less efficient use of •OH.The above results indicate that increased C=O, C-O-C functional groups, and the high humic acid content positively affect Fenton oxidation.
This study investigated the direct oxidation of all-alkanes at higher oil concentrations, and the role of hydrophilic groups in the oxidation reaction was incorporated.All these findings demonstrate compellingly the potential of hydrophilic Fe-SOM to treat high concentrations of oil-contaminated soil and the importance of hydrophilic groups in the Fenton reaction.

Figure 1 .
Figure 1.Direct Oxidation of all alkanes(C 11 -C 30 ) with fast oxidation group.(a)for 3812 and 4309 mg/kg Fe, (b) for 4728 and 5171 mg/kg Fe, (c)for 4889 and 5309 mg/kg Fe, (d)for 5129 and 5915 mg/kg Fe.Symbols and columns may cover the error bars.

Figure 2 .
Figure 2. Direct oxidation of each component alkane with fast oxidation group.Symbols and columns may cover the error bars.

Figure 4 .
Figure 4. Relationship between C=O and C-O-C absorbance and total Fe and FeOOH in the fast (a and c) and slow (b and d) oxidation groups.relationship between C=O C-O-C absorbance integral area in the fast (e) and slow (f) oxidation groups.Symbols and columns may cover the error bars.

Figure 6 .
Figure 6.Variation in initial TOC content and post-oxidation content in the fast and slow oxidation groups.Symbols and columns may cover the error bars.

Figure 7 .
Figure 7. Instant intensity of H 2 O 2 decomposition and •OH and oxidation per unit •OH of each chain hydrocarbon fraction for the fast oxidation group.H 2 O 2 decomposition for the fast oxidation group (a), instant intensity of •OH (b), H 2 O 2 decomposition rate (c), and conservation of each chain hydrocarbon component the per unit •OH (d).Symbols and bars can be overlaid with error.

Figure 8 .
Figure 8.The comparison of this study with others.Symbols and columns may cover the error bars.

Table 1 .
Distribution of iron forms in eight types of soil materials.

Table 2 .
XPS peak area of fast oxidation group and slow oxidation group.

Table 3 .
Comparison of FeOOH content, iron content, absorbance of functional groups and TPH removal rate fast and slow oxidation group.

Table 4 .
Organic component content comparison in fast and slow oxidation reaction group.
Table 5).Previous studies achieved only 27% TPH (12178 mg/kg) removal by adding 0.90 mol/L H 2 O 2 and 0.058 mol/L Fe 2+ , and the oxidation of long, medium and short-chain alkanes was not uniform.The previous study only oxidized 793.6 mg/kg of C 27 -C 30 , while the fast oxidation group oxidized 1524.79 mg/kg of C 27 -C 30 , 1.92 times more than the former (Xu et al. 2019).Usman et al. showed the same problem by dosing 0.40 mol/L H 2 O 2 and 0.040 mol/L Fe 2+ on TPH (4000 mg/kg) (Usman et al. 2012), which only oxidized 367.15 mg/kg of C 27 -C 30 , whereas the fast oxidation group oxidized 4.15 times as much.In comparison, the oxidation of long-chain alkanes (C 27 -C 30 ) in soil with Qin et al. by adding 0.240 mol/l H 2 O 2 and 0.06 mol/L Fe 3+ was only 55.52 mg/kg (Qin et al. 2021), which was 27.47 times higher in the fast oxidation group.